Lactose crystals. Polarized light micrograph of crystals of lactose monohydrate, a form of sugar (disaccharide) that is found in human and cow milk. It is formed in the mammary glands and used to feed newborn mammals. The intestinal villi secrete the enzyme lactase to digest it. Lactose intolerance is the inability to digest lactose, due to a lactase deficiency in the digestive system. Some fermenting bacteria convert lactose into lactic acid, the basis of several dairy products. Field of view size: 0.740 x 1.110 millimetres.